In such a known system, the input multiplex comprises both digital data and header information (“signaling”) serving firstly to identify said packets and secondly to specify the output to which the packet is to be routed.
Routing methods are already known, one of which relies on implementing predetermined routing tables, and another of which implies demodulating and decoding all of the information contained in the packets (data and headers).
The first known method, used in particular in the Euroskyway and Ibis systems, makes use of regeneration on board a satellite of uplink signals that are organized in frames. Each frame contains a certain number of time sub-slots, each containing an MPEG2-TS packet or an ATM cell. The processor on board the satellite contains routing tables preloaded into a memory and providing routing information corresponding to the packet in each time sub-slot of the frame. As a result, packets are routed by a switching procedure of the kind that could be performed by a circuit, i.e. there is no self-routing of the packets. The routing tables are loaded from one or more terrestrial stations (e.g. a network control center) and they can be updated every second. Those communications systems operate in a meshed network with an accumulated data rate for the system of the order of 1 gigabit per second (Gbit/s) shared between a few tens of user beams, e.g. 30 beams.
The second known method, as used in particular in the Astrolink and Spaceway systems, implements regeneration on board a satellite of packets received from an up link, and switching or routing is performed on the basis of self-routing. Reference can be made in particular to the article by W. Buerkle and M. Trefz entitled “On-board switching architectures for multimedia satellite systems” published in Space Communications 17 (2001), pages 215-229.
Such a method implies demodulating and decoding all of the information contained in the packets (digital data and headers) on board the satellite, which not only implies a large quantity of processing, but also implies a loss of flexibility, in particular concerning packet modulation and encoding. Demodulated packets are stored in a buffer memory and the headers are extracted to enable the desired routing function to be performed. When two (or more) packets have to be routed to the same downlink, a contention problem arises which is solved by putting one (or more) packets into memory while another packet is being routed to the contenting downlink. Any packets stored in the buffer memory are subsequently routed one after another to the downlink.
A switching circuit (a “crossbar” system or by a shared memory) is implemented for routing the packets to the corresponding outputs. In all cases, there exists a queue for the packets, whether at the input, in the shared memory, or at the output, depending on the technology that is implemented. This process of storing in a buffer memory leads to a delay in the switching process and might constitute a bottleneck in future satellite systems of very large bandwidth (accumulated data rates of the order of 50 Gbit/s). At the output, after routing, the packets are formatted, encoded, and modulated prior to being transmitted. Such systems are generally capable of accommodating an accumulated data rate of a few gigabits (e.g. 5 Gbit/s) which is shared over several tens of user beams (about 30), and they are compatible with multicasting, at the cost of duplicating data on board the satellite.